The invention relates to a pressurized reactor system and a method of operating the same. More precisely, the present invention relates to a method and apparatus for controlling the conditions inside a pressure vessel of a pressurized reactor system while heat derived reactions (e.g., combustion or gassification) are being practiced in the pressurized reactor system, and for simultaneously controlling the conditions in the pressure vessel.
U.S. Pat. No. 5,251,343 discloses a pressurized fluidized-bed boiler power plant, having an air cooler positioned in a duct for conveying the compressed air from the compressor to the boiler. The pressure vessel of the power plant has also a heat insulating shield inside the pressure vessel. The compressed air cooled by the cooler is introduced into the volume between the insulating shield and the inner wall of the pressure vessel and further into the volume between the shield and the reactor. The air is finally supplied to the boiler for combustion therein. This solution does not, however, give adequate active control of the temperature in the pressure vessel; the gas flow and the temperature always depend on the process requirements in the reactor, i.e., combustion.
Cooling of compressed air prior to introducing it into the pressure vessel has also been suggested in publication WO/91/17389 and U.S. Pat. No. 4,852,345. Both documents teach cooling compressed air prior to introducing it into a pressure vessel and subsequently to a reaction chamber.
It has also been suggested to provide a flow of water in the wall structure of the reaction chamber system to maintain the temperature of the reaction wall at a certain level. However, this level is usually so high that the water must be at a very high pressure, e.g. up to 5 bar, in order to avoid vaporization and maintain controllability of the temperature. Such high-pressure structures are costly, massive, and are usually undesirable.
Prior art systems still have significant drawbacks, especially with respect to active control of the conditions in the pressure vessel. Cooling of the pressure vessel by providing process gas flow through an entire gas volume of a pressure vessel provides inadequate control.
According to one aspect of the present invention, a method of operating a pressurized reactor system is provided. The reactor system includes a process vessel assembly, having a reaction chamber, enclosed within a pressure vessel, a first conduit for conveying pressurized gas to the reactor system, an inside volume within the pressure vessel defined between the interior of the pressure vessel and the exterior of the process vessel assembly, and a second conduit for conveying discharged gas from the process vessel assembly to the exterior of the pressure vessel. The method comprises the steps of: (a) Introducing superatmospheric pressure gas from the first conduit into the process vessel assembly. (b) Maintaining heat derived reactions in the reaction chamber of the process vessel assembly. (c) Exhausting gas from the process vessel assembly and pressure vessel through the second conduit. (d) Circulating gas from one part of the inside volume to another to control the temperature of the inside volume.
Step (d) is preferably practiced by circulating an inert gas, such as nitrogen or carbon dioxide, or alternatively, re-circulating the air. There is also preferably the further step of cooling or heating the circulating gas during the practice of step (d). Also, there are preferably the further steps of controlling the flow rate of the circulating gas during the practice of step (d) (as by controlling an automatically operated valve, or by controlling the speed of a fan or blower), and in increasing the pressure of the circulating gas during the practice of step (d) (as by introducing compressed gas into the recirculation loop). The heating step may be practiced during start-up, and then there is the further step, after start-up is completed, of terminating the heating of the circulating gas and subsequently cooling the circulating gas during the practice of step (d).
Step (d) may be practiced by withdrawing gas from the inside volume at a first location to pass it outside the pressure vessel; modifying the temperature of, and boosting the pressure of, the circulating gas outside the pressure vessel; and returning the circulating gas to the inside volume at a second location spaced a significant distance from the first location. Step (d) may be further practiced by withdrawing circulating gas from the top of the pressure vessel and returning the gas near the bottom of the pressure vessel (during the steady-state operating procedure), or vice-versa (typically during start-up). Alternatively, step (d) may be practiced essentially completely within the inside volume within the pressure vessel by providing one or more interior generally vertical conduits complete with the inside volume and by practicing step (d) within the interior generally vertical conduits. During the practice of step (d) the gas typically flows generally upwardly downwardly in the interior conduit by natural convection, and the temperature of the gas may be modified as it is circulating within the interior conduit.
Step (c) is typically practiced by combustion or gassification of fuel in a fluidized bed of solids, the process vessel assembly comprising a circulating fluidized bed reactor, and step (a) is typically practiced to introduce gas under pressure between 2-100 bar. Also, in response to, or in anticipation of, a shutdown of the process vessel assembly, gas may be withdrawn from the inside gas volume and introduced into the circulating fluidized bed reactor to terminate the combustion or gasification reactions therein.
Step (d) is typically practiced to modify the temperature of the circulating gas to avoid condensation of corrosive gases on, and to prevent the temperature increasing to detrimental levels in the pressure vessel and process vessel system.
According to another aspect of the present invention, a pressurized reactor system is provided comprising the following elements: A pressure vessel. A process vessel assembly within the pressure vessel, having a reaction chamber in which heat derived reactions take place. An inside gas volume defined between the interior of the pressure vessel and the exterior of the process vessel assembly. A source of superatmospheric pressure gas exterior of the pressure vessel. A first conduit for conveying gas from the source to the reaction chamber in the process vessel assembly. A second conduit for conveying gas discharged from the reaction chamber to the exterior of the pressure vessel; and means for circulating gas from one part of the inside volume to another to control the temperature of the inside volume.
The gas circulating means may include a gas passage and means for heating or cooling the gas contained within the gas passage. The gas passage is typically disposed either primarily exteriorily of the pressure vessel, or completely within the pressure vessel. Where the gas passage is disposed primarily exteriorily of the pressure vessel, means are provided for withdrawing gas from a first part of the pressure vessel and reintroducing the withdrawing gas after heating or cooling thereof into a second part of the vessel lightly spaced from the first part, at least enough to effect the desired function of the circulation means.
The system may further comprise a fan or blower disposed in the passage exteriorily of the pressure vessel for controlling the flow rate of gas circulation and acting on the gas to effect circulation thereof. There may also be provided means for boosting the pressure of the circulating gas, such as a compressor.
The process vessel assembly preferably comprises a circulating fluidized bed reactor.
A control valve is preferably provided in the first conduit, which control valve may be automatically operated to close off the supply of reaction gas to the fluidized bed reactor in emergency situations.
The circulating means may include a generally vertically extending gas passage disposed completely within the inside volume, having an opening for entry or exit of gas within the inside gas volume adjacent a bottom portion thereof, and an opening for exit or entry of gas from said passage adjacent the top thereof. The direction of gas flow through the circulation means is determined by convection due to the cooling or heating of the circulating gases. Means for heating or cooling gas circulating in the gas passage may be disposed in the gas passage, such means comprising, for example, a tube type heat exchanger or a plate heat exchanger which defines a part of the passage. The gas passage is preferably dimensioned, oriented, and constructed so that gas circulates therethrough by natural convection.
A pressure relief valve may be provided connected to the pressure vessel to vent pressure therefrom under emergency situations. Also, a third conduit is preferably provided leading from the inside gas volume to outside the pressure vessel, and then back into the reaction chamber and then the fluidized bed reactor, and an automatically operated valve is provided in the third conduit exteriorily of the pressure vessel. An automatically controlled valve is also preferably disposed in a gas passage exteriorily of the pressure vessel.
The circulating means disposed interiorily of the pressure vessel may include a generally vertically extending plate disposed entirely within the inside gas volume and spaced from, but adjacent, a vertical wall of the pressure vessel to define the gas passage which is open at the top and bottom. A flow control valve may be provided adjacent the bottom of the passage. A plurality of interior tubes or gas-passage defining plates may be provided in the pressure vessel.
When gas is introduced into one end of a circulation conduit, e.g. at the upper portion of the pressure vessel, and, when the gas is, e.g. cooled, it flows downwardly as a result of a pressure difference, which is dependent on the gas density. The pressure difference as a driving force thus results from the cooling of the gas. Thus, circulation of the gas may be provided even without a mechanical blower. A minimum requirement is to provide a flow channel between two locations in the vessel, and to connect the heat transfer means in such a manner that the temperature of the circulating gas is influenced: if the gas is heated, the flow direction is upward, and if cooled, it is downward.
It is desirable to use inert gas as the circulation gas, thereby reducing the risk of corrosion of the gas flow-defining surfaces to a minimum. The inert gas may be N.sub.2, CO.sub.2, or other available inert gas or gas mixtures. Use of inert gas gives an additional advantage: by using inert gas as the circulating gas, it is possible to utilize the circulating for emergency shutdowns. If the pressurized fluidized bed reactor system is used for pressurized gasification or combustion of fuel material, e.g., in connection with a gas turbine-compressor unit driving a generator, there is a need for quick termination of reactions in the process vessel system. If there is reason for a quick shutdown--such as by a sudden loss of turbine load--it is essential to terminate the combustion reaction in the reactor immediately for safety reasons. This may be completed very conveniently by injecting the circulating inert gas into the reaction chamber. For that purpose, the gas circulation system may be provided with a quick-connect conduit to take the inert gas into the reactor via conduits leading thereto.